Crystal contact rectifiers



July 3, 1956 A. JENKINS ET AL CRYSTAL CONTACT RECTIFIERS Filed June 50, 1952 United States Patent Office 2,753,497 Fatented July 3, 1956 CRYSTAL CONTACT RECTIFIERS Alexander Jenkins and Arthur Langridge, London, England, assignors to Westinghouse Brake and Signal Company, Limited, London, England Application June 30, 1952, Serial No. 296,316 Claims priority, application Great Britain August 3, 1951 3 Claims. (Cl. 317-436) This invention relates to crystal contact rectifiers of the kind hereinafter referred to as the kind described in which one electrode is a semi-conducting crystal, and the other electrode is a so-called cats whisker making contact with a portion of a surface of the crystal.

Hitherto it has been usual to employ a cats whisker having a very fine point presented to a highly polished surface of the crystal and, in an endeavour to keep this pointed cats-whisker in position once a sensitive spot on the crystal surface has been found, it has been proposed to enclose the whole in an insulated tube which was then filled with paraffin wax. It has also been proposed to cement the tip of the cats-Whisker in position by covering the surface of the crystal around the pointed end of the cats whisker with a thin layer of a suitable polymerisable material.

This known arrangement has the disadvantage that the necessarily delicate nature of the finely pointed catswhisker tends to render contact with a sensitive spot on the crystal surface unstable and liable to dislodgement, in spite of measures to cement the cats-whisker in position. Furthermore the area of contact between the cats-whisker and the crystal must depend, to an extent, upon the angle at which the pointed end is presented to the crystal surface. It has also been found that relatively thick layers of thermo-plastic materials of a soft character, such as polystyrene, shellac, waxes, cellulose, etc., when applied to the surface of the crystal, do not satisfactorily hold a conventionally pointed cats-whisker firmly against mechanical vibration or electrical overload. This is believed to be because, the end of the cats-whisker being conical in shape, vibration or softening of the thermo-plastic material creates a pressure against the sloping sides of the cone which ultimately pushes the tip of the cats-whisker away from the crystal surface.

It is an object of this invention to provide a crystal contact rectifier of the kind described wherein the above disadvantages are reduced or eliminated.

Accordingly to the invention in a crystal contact rectifier of the kind described, the cats-whisker is ball pointed. The surface of the crystal with which the ball point makes contact may be roughened, or the ball point may be roughened and the crystal surface polished, or both may be roughened. Alternatively, neither may be roughened and the ball-point may make contact with an untreated natural surface of the crystal.

Preferably the ball point is secured in position by a layer of insulating material of sufficient thickness at least to cover the ball point. The term insulating material used herein in this connection is intended to include, in their so-called insulating state, materials known as semiconductors, such as selenium. Preferably a thermo-plastic material is used.

The rectifier may be enclosed in a casing, a film of the thermoplastic material being caused to be deposited over the whole length of the cats-whisker and over the interior walls of the casing.

The toughening of the crystal surface may be carried out, with a germanium crystal for example by a chemical etching process such as by treating the surface with a solution of hydrogen peroxide, or alternatively, by heating the crystal in a stream of oxygen or a halogen gas, such as chlorine.

This roughening brings out the crystal structure of the surface and it is believed that, whereas the conventional cats-whisker point becomes wedged in a crevice at the surface of the crystal, thus forming a line contact, the ball point of the rectifier of the invention is supported upon two or more crystal edges which are accentuated by the etching process. Thus, although the nominal area of contact of the ball may appear to be many times that of the conventional fine point, the actual contact area be tween the ball and the crystal is not very much if at all, greater than that between the crystal and the fine point, and it is found that the ball pointed contact appears to sufifer no loss of high frequency efiiciency when compared with the conventional finely pointed contact up to a frequency of megacycles.

The ball point may be formed in any suitable manner, such as by an electric are or by an electrolytic process and is cheaper to produce than the usual finely pointed cats-whisker. A cats-whisker of 0.002 inch diameter and an effective length of 0.1 inch having a ball point of approximately 0.006 inch diameter has been found to give satisfactory results when used in conjunction with a germanium crystal. If the ball is made substantially spherical, it has the advantage that, whatever the angle of the whisker, there will be the same area of contact between the ball and any given point on the crystal surface. Furthermore, by increasing the diameter of the ball point a greater number of contacts may be made with the roughened crystal surface, thereby increasing the current carrying capacity of the rectifier and improving the thermal dissipation from the seat of the rectification.

By ensuring that the thermoplastic material covers the ball, it is ensured that the latter will be held firmly in contact with the crystal in spite of variations in temperature and mechanical vibration. The forces set up on the curved surface of the ball by the contraction of the thermo-plastic material are substantially balanced.

In an alternative arrangement the ball point surface of the ball pointed cats-whisker may be roughened by a suitable chemical etching process and the surface of the crystal with which the ball point makes contact polished, for instance by an anodic etching treatment, the roughness of the ball point advantageously providing both a small actual contact area and a key on which the thermoplastic material can bond itself.

The invention is illustrated, by way of example, in the accompanying drawing which shows in part-longitudinal cross section a crystal rectifier in which a semi-conducting crystal 2 of germanium is secured in any suitable manner to a metal holder 1, the surface of the crystal then being roughened by a chemical etching process. The crystal and holder are then inserted in one end of a tubular casing 3 of ceramic material, forming an end cap closing the casing at that end. The other end cap 5, in the form of a plug having a concentric threaded bore 4 therein, is then inserted in the other end of the tubular casing and the two end caps are secured in position. The whole is then heated to a temperature of about 100 C. and a small quantity of the polymerisable material sold under the Registered Trademark Araldite is introduced into the casing. A threaded metal electrode holder 6, to one end 9, of which has been secured the ball pointed cats-whisker 10, is then screwed into the threaded bore in the plug until a good rectifying contact is made between the ball point 11 and the crystal 2. In the drawing, the size of the ball point is somewhat exaggerated for the purpose of illustration. The unit is then heated for one hour at 150 C. in order partially to polymerise the Araldite so that the latter behaves as a thermo-plastic material. Suflicient Araldite is used to form a layer 13 thick enough to cover the ball point and the space between the crystal surface and the plug is preferably approximately half filled initially, when the material, due to a surface tension effect, formsafilm (not shown) over the whole of the interior of the casing and over the length of the cats-whisker, thus hermetically sealing the unit and supporting the cats-whisker over its entire length. in order to enable external electrical connections to be made to' the crystal and the cats whisker, the crystal. holder 1 and the electrode holder 6, each have a longitudinal hose, 12 and 7 respectively, into which can. be inserted a connection wire which can then be soldered in position.

It is to be understood that the foregoing description is given only by way of example and that the method of manufacture may be varied without departing from the scope of the invention. Crystals other than germanium, for example, and materials other than that sold under the Registered Trademark Araldite, may be used.

Having thus described our invention what we claim is:

l. A crystal contact rectifier comprising a tubular casing of insulating material; a first end cap closing one end of said casing; a second end cap closing the other end of said casing; a semi-conducting crystal mounted on said first end cap and within the casing; a ball pointed cats-whisker secured to said second end cap and with the ball point in contact with a surface of said crystal; a layer of insulating material covering said ball point and securing it in position on said surface and extending over the interior walls of the casing and the surface of the cats-whisker, thereby hermetically sealing the casing and supporting the cats-whisker, throughout its length; and means for making external electrical connection to said crystal and said cats-whisker respectively.

2. A crystal contact rectifier comprising a tubular casing of insulating material; a first end cap closing one end of said casing; a second end cap closing the other end of said casing; a semi-conducting crystal mounted on said first end cap and within the casing; a ball pointed catswhisker secured to said second end cap, a surface of the ball point being in contact with a surface of the crystal, at least one of said surfaces being roughened; a layer of partially polymerised polymerisable material covering said ball point and securing it in position on said surface and extending over the interior walls of the casing and surface of the cats whisker, thereby hermetically sealing the casing and supporting the cats-whisker throughout its length; and means for making external electrical connection to said crystal and said cats whisker respectively.

3. A crystal contact rectifier comprising a tubular casing of insulating material; a first end cap and a second end cap, each closing one end of said casing respectively; a semi-conducting crystal of germanium mounted on one of said end caps within the casing; a ball pointed cats whisker secured to said second end cap, a surface of the ball being in contact with a surface of the crystal and at least one of said surfaces being roughened; a layer of thermo-pl'astic insulating material on said surface of said crystal, covering said ball point and securing it in position on said surface and extending over the interior walls of the casing and the surface of the cats-whisker, thereby hermetically sealing the casing and supporting the cats whisker throughout its length; and means for making external electrical connections to said crystal and said cats whisker respectively.

References Cited in the file of this patent UNITED STATES PATENTS 756,676 Midgley Apr. 5, 1904 962,262 Schloemilchetal June 21', 1910 1,670,589 Martin May 22, 1928 2,406,405 Salisbury Aug. 27, 1946 2,430,028 Pfann et al. Nov. 4, 1947 2,432,116 McLean et al. Dec. 9, 1947 2,475,940 Brittain July 12, 1949 

1. A CRYSTAL CONTACT RECTIFIER COMPRISING A TUBULAR CASING OF INSULATING MATERIAL; A FIRST END CAP CLOSING OUT ENDS OF SAID CASING; A SECOND END CAP CLOSING THE OUTER END OF SAID CASING; A SEMI-CONDUCTING CRYSTAL MOUNTED ON SAID FIRST END CAP AND WITHIN THE CASING; A BALL POINTED CAT''S-WHISKER SECURED TO SAID SECOND END CAP AND WITH THE BALL POINT IN CONTACT WITH A SURFACE OF SAID CRYSTAL; A LAYER OF INSULATING MATERIAL COVERING SAID BALL POINT AND 